Is the negative effect of crowding on ingestion rate in Daphnia pulex induced physically or chemically?

Abstract

The effect of physical interference and accumulation of auto-metabolites due to crowding on ingestion rates in Daphnia pulex were examined under an excess food supply. Ingestion rates of D. pulex decreased to 40–80% with an increasing number of animals in an experimental container irrespective of the volume of the container. Conditioned water from the crowded culture of D. pulex did not affect the ingestion rate. These results suggest that physical interference may play a more influential role in the detection of crowding rather than the accumulation of auto-metabolites. Since the daphniids did not visually respond to crowding, the effect of crowding on daphniid feeding shown in this study is considered to be caused by some physical interaction, e.g. physical contact between neighbouring individuals or pulses generated by the beating of thoracic limbs of other individuals.